You can find relatively cheap audio transducers for very little money (look for Dayton Audio's Exciters). I used them a lot for various projects, last time was to make a plate reverb, they work really well for the price.
I can't really see how Nicola Ratti is using it in this video though, it looks as if it's directly in contact with the strings..
You can actually use a piezo disc as a transducer (just add an audio transformer to adjust impedance on the output of a small amp), it can work pretty well, depending on the surface it's attached to, I used it on some resonant objects for a sound installation two years ago. Nicolas Collins explains it in his book "Handmade Electronic Music" :
http://www.nicolascollins.com/handmade.htm (he also explains you can use small motors, vibrators from cell phones etc.).
If you want to work with strings though you might want to drive an induction coil and use a small neodyme magnet to create an electromagnetic field and drive the string/object present in between. It's the same principe as an eBow, but an eBow works by feed backing an electromagnetic pickup and driver, here you can send whatever signal you want in the string. Be careful though you need to match your impedances and maybe add a fixed resistor depending on the power of the amplifier you use.
I learned these techniques during a workshop with media/sound artist Aernoudt Jacobs, but my notes with the technical details are far away from me right now.
Lastly, if you're into Eurorack, take a look at Pulplogics' DRVAC Tile : http://pulplogic.com/product/drvac/